The emergence of vertical-axis wind turbine generators overcomes the shortcomings of horizontal-axis wind turbine generators in their physical structure. Yet, how to increase the wind capture area and improve the wind energy utilization factor still remains the core issues in the further development of wind power.
In an existing vertical-axis wind turbine operating with airfoil-shaped sails, a plurality of airfoil-shaped sails are arranged on a rotatable annular operating platform. Such wind turbines are capable of capturing wind energy from various angles and so can increase the wind capture area.
Traditional airfoil-shaped sails have a good wind capturing capability, but they cannot address the wind resistance. A ramjet-type airfoil-shaped sail not only retains the traditional airfoil-shaped sail's feature of large wind capture area, but a head and surface air ports design adopted by the ramjet-type airfoil-shaped sail also effectively reduces the wind resistance experienced by the sail. Partial resistance is converted into a motive force to facilitate the forward motion of the sail in the direction of the sail head, greatly enhancing the sail's wind capturing ability.
FIG. 1 shows a prior art ramjet-type airfoil-shaped sail, which employs the working principle of a jet wing. A good wing should be able to produce much lift and little resistance, and should also have sufficient strength and rigidity—hence invulnerable to deformation. A good wing should also be able to be easily controlled. There are many factors that determine the amount of lift generated by a wing, e.g., the lift can be directly related to the wing area, wind speed, etc. Such factors, however, usually cannot be or cannot be easily changed. For example, the air density cannot be changed; the wing area is usually subject to physical constraints; and the oncoming airflow velocity in the natural environment where the wing is placed is practically uncontrollable. Therefore, the goal of increasing the lift can only be achieved by increasing the lift coefficient. This is also the way to reduce the drag on the wing—mainly by managing to reduce the drag coefficient of the wing. The wing's lift and drag coefficients are determined by the wing's cross-sectional shape (i.e., airfoil type), the wing planform, and the then angle of attack. A good airfoil type can have a large lift coefficient as well as a small drag coefficient with respect to the same angle of attack, and the ratio of the two coefficients (called lift-to-drag ratio) can reach up to 18.
On the other hand, when its leading edge faces the wind, an airfoil-shaped sail yields the highest wind energy utilization factor, which decreases to certain degrees at other angles the sail faces the wind. Such airfoil-shaped sails, however, cannot make adaptive adjustments in response to different wind speeds and different angles of attack, resulting in a low average wind energy utilization factor.
China Patent Application No. ZL201080047198.9 had disclosed a water-floating sail wind turbine, which employs an air-channel array design to improve the lift-to-drag ratio. A plurality of ramjet-type airfoil-shaped sails are arranged over a water-floating operating platform to serve the function of capturing wind energy. Each ramjet-type airfoil-shaped sail includes a sail head, a sail tail, a group of head air inlets provided at the sail head, and an array of surface air channel ports defined in each of the two sail surfaces. The head air-inlet group is composed of a series of air-inlet ports located in the sail head. The surface air-channel port arrays are composed of surface multipolar air-channel ports distributed at the sail's two airfoil surfaces. Thus, the head air inlets and surface multipolar air channel ports form air jet channels inside the ramjet-type airfoil-shaped sail. As such, during the horizontal circular motion in different directions, the ramjet-type airfoil-shaped sails that are evenly distributed on the water-floating operation platform can always generate a favorable resultant force facilitating the forward motion with respect to different angles of attack.
But none of the above-mentioned prior arts solves the following problems.
First, no single airfoil-shaped sail is able to synchronously adjust the flow guidance capacity of the jet channel in response to the airflow direction, speed, and force in the wind field where the sail is situated, to obtain the maximum lift-to-drag ratio and the optimal resultant force facilitating the sail's circular motion.
Second, no single airfoil-shaped sail is capable of adaptive regulation of its jet channel through the jet channel cross-sectional shape in combination with the number of jet channels, to obtain the maximum lift-to-drag ratio and the optimal resultant force facilitating the sail's circular motion.
Third, the adaptive regulation of the jet channel relies on a certain regular control to obtain the maximum lift-to-drag ratio and the optimal resultant force facilitating the sail's circular motion.
Fourth, considerations are not taken as to how to select the number of airfoil-shaped sails and arrange the position of each single sail, as well as how to enable each sail to adaptively adjust itself, to facilitate the whole wind turbine composed of a plurality of sails to achieve the optimal resultant force facilitating its circular motion.
Therefore, in-depth studies are yet still required on how to enable each airfoil-shaped sail to make synchronous and flexible adaptive adjustments to its own structure in response to the wind direction and speed in the wind site, so as to obtain the maximum lift-to-drag ratio and the optimal resultant force facilitating the sail's circular motion, maximizing the wind energy utilization factor.